Various forms of known apparatus use vibratory methods to measure properties such as viscosity of a material. These all operate on the general principle of bringing the said material into contact with a member, generating and possibly maintaining vibrations on this member by exciting means, detecting the influence of said material on the characteristics of these vibrations and of inferring properties of said material by analysis of this influence on the characteristics of the vibrations. The exciting means for generating and possibly maintaining vibrations do not include the material under test but rather the material only influences the characteristics of vibrations generated by other means and in general these exciting means are located separately from said material. In many cases the material is contained within a sealed chamber and the exciting means are attached to the vibrating member on the outside of the chamber. In general, the vibrations generated on the member by the exciting means are chosen to occur at a limited number of well-defined frequencies. GB-A 1295617 is an example of such known apparatus. Vibrations are induced in a paddle by an electromagnet acting on an arm connected to the paddle by means of a spring and the vibrations induced are detected by a further electromagnet connected to a further arm forming an extension of the first arm on the other side of the spring. The electromagnets and the arms are contained within a sealed housing, the spring extending through a sealable connection to the paddle. The apparatus is designed to operate at the specific natural resonant frequencies of the vibrating system.
Known apparatus as described above is unsuitable for measuring properties of a process material either as it is being agitated within a stirred vessel or as it is being pumped through a pump housing, for two main reasons. Firstly, the vibrating member would be excited, not only by the external means, but also by the flow of the material within the vessel or the pump and this would swamp the vibrations generated by the external means, making it difficult to detect the influence of said material on the characteristics of these externally-generated vibrations, hence making it difficult to infer properties of said material. Secondly, sealing means would be required around the apparatus as it pierces the wall of the vessel or pump and this would be a serious disadvantage in most cases involving materials that are either hazardous or must remain sterile. Known apparatus for use in stirred vessels comprise non-vibratory sensors such as thermometers, pH probes and dissolved-oxygen probes and such sensors are used as a means of monitoring and, optionally, controlling the process more effectively with the aim of improving either the process yield or the quality of the finished product. While these devices may proved adequate within certain process regimes, they may be insensitive to some changes in material properties that have a major influence on yield and product quality. Such non-vibratory sensors also suffer from the drawback of requiring sealing means where they enter a fully-enclosed vessel. Many batch processes within stirred vessels; for example resin manufacture, polymerisation and biotechnology reactions; undergo significant changes in viscosity through the batch and viscometry is therefore a potentially useful means of monitoring and, optionally, controlling the process. However, current techniques for this purpose include either sampling the material and using instruments such as rotating cups or U-tubes or pumping the material to a sealed rotating-cup device. The former practice is time-consuming for repeated measurements and requires a leak-proof sampling mechanism while the latter suffers from possible problems of clogged pipework and cross-contamination between batches.
Viscometry at non-ambient temperatures, whether the material has been sampled from a stirred vessel or not, is difficult or inconvenient using existing means, because the whole of the apparatus in contact with the material must be maintained at the required temperature.
While in some instances it might be possible to infer changes in viscosity from corresponding changes in power absorbed by the material (at a constant rotational speed), this is only possible at low and intermediate values of Reynold's number, in which viscous forces are significant. Thus, known art includes means to measure the absorbed power, either by recording the current drawn by the driving motor or by recording the torque on the shaft of by some other means, and hence infer changes in viscosity at low and intermediate values of Reynold's number. GB-A-764 850 for example describes measurements taken with a driving member which can be constantly rotated. However these measurements are not taken from the driving member during its constant rotation: they are either taken from a separate measuring member or from the driving member when it is no longer driven at constant speed.
At high Reynold's number, where interia forces predominate over viscous forces, little or no change in power at constant speed can be detected as viscosity varies. Since many stirred vessels operate at high Reynold's number (to achieve the turbulence required to enhance mixing), there is a clear need for means and apparatus to monitor viscosity changes in this region.